Al2O3 — TiO2 dispersion-strengthened granular refractory ceramics

Abstract

The technology, properties, phase composition, and structure of Al 2 O 3 -TiO 2 refractory granular ceramics are reported. Additional heat treatment provides conditions for generating nanocrystals and increasing significantly the ceramic strength. A dispersion-strengthening mechanism for refractory ceramics is proposed.Refractory ceramics, owing to their high-strength properties, have found application in protecting engineering components from mechanical impact. The most advantageous for that purpose are porous refractory ceramics with a nonuniform granular structure which provide for a rapid decay of stresses generated by impact loading.Ceramics in systems Al 2 O 3 -TiO 2 , Al 2 O 3 -Al 2 TiO 5 etc. have been developed and their properties studied [1 -4]. Aluminum titanate used as a component of composite ceramic materials was shown to improve their thermomechanical properties [5 -7].Data have been reported [3,4] showing that the strengthening of ceramics occurred because of the decomposition of Al 2 TiO 5 subjected to further heating which resulted in the formation of a fine crystalline structure, an elastic stressed state, and partial healing of microcracks. Maximum strengthening was achieved with complete decomposition of Al 2 TiO 5 . The ceramic specimens were prepared using different techniques and heating regimes [1 -4]; still, in all instances a tendency towards increasing mechanical strength was observed, mostly owing to the production of finely dispersed Al 2 O 3 and TiO 2 phases.Our goal was to develop a technology for preparation of porous granular ceramics in the system 75 wt.% Al 2 O 3 + 25 wt.% TiO 2 . The technology in question involved two steps. In step 1, a materials was synthesized to prepare a composition of the major components Al 2 O 3 , Al 2 TiO 5 , and TiO 2 . In step 2, granular components were used to mold tests specimens under pressure; these were burnt for imparting strength to them.The precursor constituents used were aluminum oxide, reagent-grade, not less than 95% Al 2 O 3 , and titanium dioxide, P-O2 pigmental grade, 90 -93% TiO 2 . Aluminum oxide in phase composition was represented by a-Al 2 O 3 with minor amounts of partly amorphized g-Al 2 O 3 and q-Al 2 O 3 . Morphologically, aluminum oxide was represented by compact spherulites 15 -45 mm across; titanium oxide -as particles less than 1 mm in size.In step 1 (granular ceramic preparation), Al 2 O 3 and TiO 2 , taken in the required proportion, were dry-mixed and pelletized by semi-dry pressing under a pressure of 50 MPa (the bond was a 5% polyvinyl alcohol solution) and heat-treated at 1300 -1350°C and holding time of 2 -3 h. Major crystalline phases identified in sintered specimens by x-ray phase analysis were (at 1300 -1350°C, respectively), vol.%: Al 2 TiO 5 , 18 -39; a-Al 2 O 3 , 50 -49; and TiO 2 , 17 -3. In the temperature range of 1300 -1350°C, two processes presumably occur: (i) formation of Al 2 TiO 5 and (ii) the early stage of its decomposition, of which the appearance of an amorphous phase (some 15 vol.%) at 1300°C...

Strengthened porous ceramic based on mullite and corundum

“…For example, results are provided in [1] for studying the possibility of creating high-strength porous material based on the system Al 2 O 3 -TiO 2 by synthesis, and then breakdown of the chemical compound Al 2 TiO 5 during heat treatment. Very fine (nanosize) aluminum and titanium oxides released during its breakdown form as a result of sintering at 1200 -1300°C a high-strength ceramic bond that makes it possible with open porosity of 20% to have an ultimate strength in compression reaching~500 MPa.…”

“…The HA compound was used in the form of coarse-ground material with a particle size of 50 -100 mm. An additional strengthening addition to the compound charge was 2% (above 100%) finely-dispersed mixture containing 65% SiC and 35% MgO [2]. The mixed powder particle size was~2 mm.…”